Though nitrides are inferior to oxides in stability or facilitation of production, not a few of them are known to have characteristics which oxides or other inorganic compounds do not have. Actually, such binary system nitrides as Si3N4, BN, AlN, GaN and TiN are used for various purposes such as substrate material, semiconductor, light-emitting diode (hereinafter abbreviated as “LED” as appropriate), structural ceramics and coating agent, and in industrial-scale production. In addition, large numbers of new substances of ternary or multinary nitrides are in production in these years. Among them, particularly phosphor materials with superior characteristics, which are made of multinary oxynitrides or multinary nitrides based on silicon nitride, have been developed recently. It is known that these phosphor materials emit yellow to red light when excited by a blue LED or near-ultraviolet LED. By such combination of a blue or near-ultraviolet LED and these phosphors, a light emitting device emitting white light can be fabricated.
A white light, which is frequently used in the uses for the illuminating device and display, is generally obtained by mixing blue, green and red light emissions in accordance with the additive color mixing principle of light. In the back-lighting of color liquid-crystal display, a field of uses for displays, it is preferable that each of blue, green and red luminous bodies has as high emission intensity as possible and good color purity, in order to reproduce colors having wide range of chromaticity coordinates efficiently. As an example of the indication of these characteristics, NTSC is known, which is a standard of color reproduction range of TV. In these years, a semiconductor light emitting device has been tried to be used as a light source for these three colors of blue, green and red. However, when semiconductor light emitting devices are used for all of the three colors, usually, a circuit for compensating a color shift is required. Therefore, it is practical to obtain desired colors of blue, green and red by the wavelength conversion of light which is emitted from a semiconductor luminous element using a wavelength conversion material such as a phosphor. Specifically, methods are known, in which blue, green and red lights are emitted using a near-ultraviolet semiconductor luminous element as the light source, and in which blue light from the semiconductor luminous element is used just as it is and green and red lights are obtained by the wavelength conversion using phosphors.
Among these three colors of blue, green and red, green is particularly important compared to the other two colors because it is especially high in luminosity factor to human eyes and it greatly contributes to the entire brightness of the display.
However, the existing phosphors emitting green lights are insufficient in conversion efficiency of blue or near-ultraviolet light and in color purity, and therefore a high-performance phosphor emitting green fluorescence (hereinafter, referred to as “green phosphor” as appropriate), which can satisfy the above requirements, has been desired.
As conventional known arts of green phosphor, the following phosphors can be cited such as SrSi2O2N2:Eu, CaSi2O2N2:Eu and BaSi2O2N2:Eu, shown in Patent Documents 1 and 2, for example. These phosphors, having emission peak wavelengths ranging from 490 nm to 580 nm by an excitation light of 400-nm wavelength, show colors of blue green to yellow red. However, as is evident from the emission spectrum of these phosphors, their peak wavelengths are too short, and color purities are too low due to the broad full width at half maximums. Thus, the color reproduction ranges of displays using these phosphors are narrow. In addition, the phosphors are low in brightness and thus they are insufficient for the above requirements.
In Patent Document 3, an oxynitride phosphor having luminescent color of blue green to yellow is disclosed. However, only the phosphors are disclosed in Patent Document 3, of which compositions are the same as those described in the above-mentioned known documents 1 and 2, or of which metal element or silicon is merely substituted by the other elements, and therefore they also have the same problems as described above.
In Patent Document 4, an Si—O—N based phosphor containing Al is disclosed. However, the phosphor is also low in color purity due to its broad full width at half maximum of the emission spectrum, which is still insufficient for the above-mentioned requirements.
On the other hand, in Non-Patent Document 1, synthetic methods of Ba3Si6O9N4 and Eu3Si6O9N4, as well as their X-ray structural analysis results are disclosed. However, none of these oxynitrides does not emit fluorescence. The document was contributed on Nov. 17, 2005 and fully disclosed online on May 3, 2006, but only its title, including such composition formulae as Ba3Si6O9N4 and Ba3Si6O9N4, was disclosed before the priority date of the present application.    Patent Document 1: Japanese Patent Laid-Open Publication (Translation of PCT Application) No. 2005-530917    Patent Document 2: Japanese Patent Laid-Open Publication (Translation of PCT Application) No. 2006-503431    Patent Document 3: The pamphlet of International Publication No. 2004-039915    Patent Document 4: Japanese Patent Laid-Open Publication (Translation of PCT Application) No. 2005-529229    Non-Patent Document 1: Z. Anorg. Allg. Chem., 2006, vol. 632, pp. 949-954